Metals have been extensively in medical devices due to their high stiffness and strength. Metal implants, being much stiffer than tissue, may become the primary load-bearing member thereby protecting the tissue from stress, which may result in undesirable stress shielding.
The use of absorbable polymers, materials that degrade in the body and then are either absorbed into or excreted from the body, has the potential to help alleviate the negative effects of stress shielding. Additionally, absorbable polymers are widely used due to their biocompatibility, and sustained release. Biodegradable polymeric materials have been used as medical devices in the form of pins, rods, anchors, screws, staples, and fasteners for a variety of medical applications. However, the relatively low stiffness and strength of biodegradable devices compared with metallic implants in most cases has typically limited their use to low-load bearing applications or non-load bearing applications.
Inorganic fillers (inclusions of ceramics or metal fibers, whiskers, platelets, or particles) have been used as reinforcement materials to enhance the mechanical properties of biodegradable polymeric materials. The embedding of such inclusions in a host matrix to make composites, which gives material properties not achieved by either phase alone, has been a common practice for many years. Using this approach, strength and stiffness of the absorbable polymers can be improved. For example, it is known that polylactides reinforced with tricalcium phosphates exhibit increased stiffness, but are brittle since no coupling or bonding has been developed between the inclusions and the polymer matrix. Hence, the polymer's ductile nature is lost. Due to such inherent brittleness, inorganic filler-reinforced biodegradable polymers have often been limited to non- or low-load bearing applications.
Processing techniques have been developed to allow nano-sized (1-1000 nm) inclusions to be introduced into non-absorbable polymers with some success. Non-absorbable polymer nanocomposites have been shown to have improved properties over polymers with micron-sized inclusions.
Various coupling agents have been utilized to improve the affinity of absorbable polymers to fillers. These coupling agents include silane coupling agents, zirconyl salts, and organic isocyanates. However, the grafted organic molecules mentioned above are typically noxious. Therefore, it would be highly desirable to form absorbable polymer composites with nano-sized (1-100 nm) inclusions where to the polymer properties are maintained, especially the polymer's ductile properties of toughness and elongation-to-break.